Individuals have an appearance (say brown eye color). Individuals also have genes. Each individual carries two copies of each of their genes, one from each parent. A gene often comes in two flavors (such as a gene for eye color that makes eyes brown or blue). An individual can thus have two copies of the gene with the same flavor (brown-brown), or two copies of different flavors (brown-blue). Human eye color is controlled by at least three genes, though we only understand two of them well.

Let's pretend that two genes (with two flavors each) explain the inheritance of human eye color (though we know that the situation is more complex than this). These two genes are a brown-blue gene and a green-blue gene. We can ask what children different combinations of parents would produce. For example, what happens when blue eyed and brown eyed parents have children?

Note that this two gene model does not explain most human eye color inheritance. We know that there are black eyes, grey eyes, hazel eyes, and differing shades of brown, blue, and green eyes that are not explained by these two genes. Human eye color inheritance is a complicated polygenic system than we are pretending in this simplified two gene example.